Translation Formulae for Non-repeatable Events.

This article reports that in Sweden period total fertility has risen continuously since the mid-1980s. Although similar trends can be found in other West European countries, the case of Sweden is remarkable: total fertility is relatively high, and the increase has been fairly steep. Translation methods cannot be applied to study parity-specific fertility, mortality, or first marriage rates, because these methods were devised to investigate repeatable events, such as age-specific fertility irrespective of parity. The salient feature of rates relating to repeatable events is that they can be simply totalled, both when cohorts and periods are studied, to yield indices of quantum. An analytical treatment of the reciprocal relations between cohort and period indices, known as translation methods, was provided by researcher N.B. Ryder some 30 years ago. His work has shed considerable light on the relationship between quantum and tempo for both cohort and period studies. These methods make it possible to isolate the impact on period quantum of changes in cohort tempo and changes in cohort quantum.